I think the Geneva conventions were also something rather new because biological warfare, civilian hostages including women and children, massacres, and destruction of vital resources like food and water were pretty standard for thousands of years of war and combat.
Of course as history has shown, no one actually bothers to follow the Geneva conventions when they face zero consequences but will totally complain if anyone else doesn't (cough Israel cough).
Biological weapons, for the time being, are mutually banned because disease is hard to control in a warzone where anything has the chance to mutate or evolve. Gas attacks are used exclusively against civilians because every army has gas masks. Although iirc Iraq used it to create a massive untraversable barrier against Iran. Otherwise everything is apparently still the same.
